PROJECT SUMMARY Formation and accumulation of aggregates in a fibrillar form - also referred as amyloid and amyloid-like fibrils- are often associated with cellular impairment and cell death. This is the case for Huntington's disease and other CAG repeat disorders where the presence of an abnormally long polyglutamine (polyGln) sequence in the disease protein leads the protein to aggregate and form neuronal inclusions. Although linked to the enhanced ability of long polyGln sequences to self-associate to form oligomers and aggregates, the exact mechanism by which these molecules promote neurodegeneration is still unclear. As of today, there are three schools of thought: aggregates are non-toxic and in fact beneficial and protective for the cell;aggregates are toxic only as oligomers/growth intermediates;and aggregates are just an epiphenomena. Since our long term goal is to find a cure for these CAG repeat diseases it is important to have a better understanding of the disease mechanism. Our preliminary data indicate that we possess a high-throughput screening assay that can identify small chemical compounds capable of modulating polyGln aggregation, the resulting formed and stabilized aggregates presenting structures very different from a regular aggregate. Based on our hypothesis that the toxicity of a polyGln aggregate depends on its structure and from our preliminary data, we propose to: (1) use the high-throughput assay to screen more compound libraries and identify additional polyGln aggregation modulators. Dose-response-curve experiments will be conducted in order to validate, rank and optimize the new hits;(2) employ biophysical methods such as light scattering, amyloid dye binding, sedimentation experiments coupled with reverse phase liquid chromatography, Fourier transformed infra-red, and electron microscopy to characterize the effect of an optimized compound on the aggregation reaction and on the aggregate structure;(3) conduct cellular experiments to test if there is a correlation between the various aggregate structures, modified or stabilized by polyGln aggregation modulators, and the cytotoxicity effect. In one case, modified aggregates will be introduced into cells, in the other case, cells expressing, after induction, the N-term part of huntingtin (disease protein in Huntington's disease) containing regular or pathological polyGln sequence will be exposed to compounds to be tested. NARRATIVE: Huntington's disease is one of the eight inherited neurological disorders that are caused by a DNA abnormality: the presence of additional CAG trinucleotide repeats in the disease gene. Huntington's disease, for which no cure exists, is a fatal illness causing involuntary movements and cognitive decline and strikes generally in mid-life. Its prevalence is about 10 cases per 100,000 people in the USA and, unfortunately, each child of an affected parent has a 50% chance of developing the disease. In this proposal we will study one of the disease mechanisms, termed aggregation, which is thought to be a major player in the disease progression. Our results will help make advances in the understanding of the pathology and also in the treatment of Huntington's disease patients.